Radio frequency (rf) ranging in propagation limited rf environments utilizing aerial vehicles

ABSTRACT

The embodiments described herein provide ranging and location determination capabilities in RF-opaque environments, such as a jungle, that preclude the use of Global Positioning System (GPS) and/or laser ranging systems, utilizing transponders and Global Positioning System (GPS) receivers located on aerial vehicles. The aerial vehicles operate above the RF-opaque environment, and communicate with a ranging device within the RF-opaque environment on frequencies that propagate in the RF-opaque environment. The ranging device transmits RF signals to the transponders, which are received by the transponders and re-broadcasted back to the ranging device on a different frequency. The aerial vehicles also provide their coordinates to the ranging device using their GPS receivers. The ranging device uses information about the transmitted and received RF signals and the GPS coordinates of the aerial vehicles to calculate a perpendicular distance to a property line from the ranging device, and/or to calculate a coordinate location of the ranging device.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application claims priority to, and thus the benefit of anearlier filing data from, U.S. Provisional Patent Application No.62/847,992, filed on May 15, 2019 and titled “USE OF DRONES ANDAUTONOMOUS VEHICLES IN RF RANGING”, the entire contents of which arehereby incorporated by reference.

FIELD

This disclosure relates to the field of RF ranging, and in particular,to implementing RF ranging in environments that often limit thepropagation of RF, such as in high density foliage.

BACKGROUND

Outdoor survey work is often performed to locate the boundaries of aproperty. With the implementation of the Global Positioning System(GPS), this survey work has been made easier in most environments withsurveyors utilizing GPS enabled receivers. However, some outdoorenvironments are RF-opaque to the frequencies that are in use in the GPSsystem, which operates at 1575.42 Megahertz (MHz) and 1227.60 MHz. Oneexample of an outdoor environment that GPS does not perform well in is ajungle. In a jungle environment, the dense foliage can preclude the useof GPS, which makes the survey work more challenging. A jungle can alsopreclude the use of laser-based ranging, due to line-of-sight issues.Thus, it is desirable to implement ranging and location determinationcapabilities in these types of environments, as it improves the accuracyand the speed at which a survey can be performed.

SUMMARY

The embodiments described herein provide ranging and locationdetermination capabilities in RF-opaque environments, such as a jungle,utilizing aerial vehicles that include transponders and GlobalPositioning System (GPS) receivers. The aerial vehicles operate abovethe RF-opaque environment, and are capable of determining their positionvia GPS. A ranging device operating on the ground in the RF-opaqueenvironment transmits RF signals to the transponders using 30 Megahertz(MHz) to 1 Gigahertz (GHz) RF that can propagate through the RF-opaqueenvironment, which are received by the transponders on the aerialvehicles and re-broadcasted back to the ranging device on a differentfrequency. The use of different transmit and receive frequencies by theranging device allows the ranging device to analyze the RF signalswithout incurring transmission/receive overlap that may occur using asingle frequency and a short ranging distance. The ranging device usesinformation about the transmitted and received RF signals to determinedistances to the aerial vehicles, which, along with the GPS locationinformation of the aerial vehicles derived from their GPS receivers,allows the ranging device to determine its location in the RF-opaqueenvironment or a distance to some other object, such as a property linein the RF-opaque environment.

One embodiment comprises an apparatus that includes a range detector andat least one aerial vehicle. The range detector transmits RF signals ata first carrier frequency (f1), and receives RF signals at a secondcarrier frequency (f2) that is different than f1, where f1 and f2 areselected from frequencies of 30 MHz to 1 GHz. The at least one aerialvehicle includes a GPS receiver and a transponder, and operatesproximate to first and second known coordinates of a property line. Theat least one aerial vehicle determines its coordinates utilizing the GPSreceiver, and provides its coordinates to the range detector. The rangedetector receives first coordinates of the at least one aerial vehiclein response to the at least one aerial vehicle operating proximate tothe first known coordinates of the property line, broadcasts a first RFsignal at f1, receives a first RF rebroadcast at f2 of the first RFsignal from the transponder, and calculates a first distance from therange detector to the at least one aerial vehicle based on the first RFsignal and the first RF rebroadcast. The range detector receives secondcoordinates of the at least one aerial vehicle in response to the atleast one aerial vehicle operating proximate to the second knowncoordinates of the property line, broadcasts a second RF signal at f1,receives a second RF rebroadcast at f2 of the second RF signal from thetransponder, and calculates a second distance from the range detector tothe at least one aerial vehicle based on the second RF signal and thesecond RF rebroadcast. The range detector calculates a perpendiculardistance to the property line based on the first and second distance,the first and second coordinates of the at least one aerial vehicle, andthe first and second known coordinates of the property line.

Another embodiment comprises a method of determining a perpendiculardistance to a property line. The method comprises operating at least oneaerial vehicle proximate to first known coordinates of a property line,where the at least one aerial vehicle includes a transponder and a GPSreceiver, determining first coordinates of the at least one aerialvehicle utilizing the GPS receiver, broadcasting a first RF signal at afirst carrier frequency (f1), wherein f1 that is selected fromfrequencies of 30 MHz to 1 GHz, receiving a first RF rebroadcast of thefirst RF signal from the transponder, wherein the first RF rebroadcastis at a second carrier frequency (f2) that is different from f1, whereinf2 is selected from frequencies of 30 MHz to 1 GHz, and calculating afirst distance to the at least one aerial vehicle based on the first RFsignal and the first RF rebroadcast. The method further comprisesoperating the at least one aerial vehicle proximate to second knowncoordinates of the property line, determining second coordinates of theat least one aerial vehicle utilizing the GPS receiver, broadcasting asecond RF signal at f1, receiving a second RF rebroadcast at f2 of thesecond RF signal from the transponder, calculating a second distance tothe at least one aerial vehicle based on the second RF signal and thesecond RF rebroadcast, and calculating a perpendicular distance to theproperty line based on the first and second distance, the first andsecond coordinates of the at least one aerial vehicle, and the first andsecond known coordinates of the property line.

Another embodiment comprises a range detector and at least one aerialvehicle. The range detector transmits RF signals at a first carrierfrequency (f1), and receives RF signals at a second carrier frequency(f2) that is different than f1, where f1 and f2 are selected fromfrequencies of 30 MHz to 1 GHz. The at least one aerial vehicle includesa GPS receiver and a transponder, where the at least one aerial vehicleis configured to determine its coordinates utilizing the GPS receiver,and to provide its coordinates to the range detector. The rangedetector, in response to the at least one aerial vehicle operating ateach of a plurality of different locations, receives coordinates of theat least one aerial vehicle, broadcasts an RF signal at f1, receives anRF rebroadcast at f2 of the RF signal from the transponder, andcalculates a distance from the range detector to the at least one aerialvehicle based on the RF signal and the RF rebroadcast. The rangedetector determines its coordinates based the distance calculated ateach of the plurality of different locations.

Another embodiment comprises a method of determining coordinates of arange detector in an illustrative embodiment. The method comprisesoperating at least one aerial vehicle at each of a plurality ofdifferent locations, where the at least one aerial vehicle includes atransponder and a GPS receiver. In response to the at least one aerialvehicle operating at each of the plurality of different locations, themethod comprises performing the steps of: receiving, by a rangedetector, coordinates of the at least one aerial vehicle utilizing theGPS receiver, broadcasting, by the range detector, a RF signal at afirst carrier frequency (f1), wherein f1 that is selected fromfrequencies of 30 MHz to 1 GHz, receiving, by the range detector, a RFrebroadcast of the RF signal from the transponder, wherein the RFrebroadcast is at a second carrier frequency (f2) that is different fromf1, wherein f2 is selected from frequencies of 30 MHz to 1 GHz, andcalculating, by the range detector, a distance from the range detectorto the at least one aerial vehicle based on the RF signal and the RFrebroadcast. The method further comprises determining, by the rangedetector, its coordinates based the distance calculated at each of theplurality of different locations.

The features, functions, and advantages that have been discussed can beachieved independently in various embodiments or may be combined in yetother embodiments, further details of which can be seen with referenceto the following description and drawings.

DESCRIPTION OF THE DRAWINGS

Some embodiments of the present invention are now described, by way ofexample only, and with reference to the accompanying drawings. The samereference number represents the same element or the same type of elementon all drawings.

FIG. 1 is a block diagram of a RF ranging system in an illustrativeembodiment.

FIG. 2 is a flow chart of a method of determining a perpendiculardistance to a property line in an illustrative embodiment.

FIG. 3 illustrates additional steps of the method of FIG. 2 in anillustrative embodiment.

FIG. 4 illustrates an estimated distance between the range detector ofFIG. 1 and the transponder of FIG. 1 in an illustrative embodiment.

FIG. 5 illustrates an example of a phase difference between the RFsignal of FIG. 1 and the RF rebroadcast of FIG. 1 in an illustrativeembodiment.

FIG. 6 is a block diagram of the RF ranging system of FIG. 1 in anotherillustrative embodiment.

FIG. 7 is a block diagram of the RF ranging system of FIG. 1 in anotherillustrative embodiment.

FIG. 8 is a block diagram of the RF ranging system of FIG. 1 in anotherillustrative embodiment.

FIG. 9 is a flow chart of a method of determining a perpendiculardistance to a property line in an illustrative embodiment.

FIG. 10 is a flow chart of a method of determining the coordinates of arange detector in an illustrative embodiment.

DETAILED DESCRIPTION

The figures and the following description illustrate specificillustrative embodiments. It will be appreciated that those skilled inthe art will be able to devise various arrangements that, although notexplicitly described or shown herein, embody the principles describedherein and are included within the contemplated scope of the claims thatfollow this description. Furthermore, any examples described herein areintended to aid in understanding the principles of the disclosure, andare to be construed as being without limitation. As a result, thisdisclosure is not limited to the specific embodiments or examplesdescribed below, but by the claims and their equivalents.

FIG. 1 is a block diagram of an RF ranging system 100 in an illustrativeembodiment. In this embodiment, RF ranging system 100 includes a rangedetector 102 that determines a perpendicular distance 124 to a propertyline 120 using two or more transponders 116-117 that are located onproperty line 120. Transponders 116-117 are separated from each other onproperty line 120 by a known distance 126. Transponder 116 is separatedfrom range detector 102 by a distance 122, which is measured by rangedetector 102. Transponder 117 is separated from range detector 102 by adistance 123, which also is measured by range detector 102. In thisembodiment, range detector 102 calculates a perpendicular distance 124to property line 120 based on distances 122-123 and known distance 126using a trigonometric relationship between the two right triangles thatare formed.

In some embodiments, RF ranging system 100 utilizes Very High Frequency(VHF) ranges (e.g., RF ranges between 30 MHz to 300 MHz as defined bythe International Telecommunication Union (ITU)) that more easily travelthrough RF-opaque environments, such as those found in a jungle. Inaddition to or instead of, RF ranging system 100 may also utilize lowerfrequency portions of Ultra High frequency (UHF) ranges (e.g., a lowerfrequency portion of RF ranges between 300 MHz to 3 GHz as defined bythe International Telecommunication Union (ITU)). In particular, RFfrequency ranges up to about 1 GHz may provide sufficient propagationperformance in RF-opaque environments, such as those found in a jungle.With respect to UHF ranges in the United States, frequencies between 420MHz and 450 MHz (i.e., the 70 centimeter (cm) band) and frequenciesbetween 902 MHz and 928 MHz (i.e., the 23 cm band) may providesufficient propagation through dense foliage while simplifying licensingrequirements with the Federal Communication Commission (FCC) of theUnited States.

During a survey, utilizing VHF ranges (or lower frequency portions ofUHF ranges) when utilizing RF ranging system 100 allows the surveyor toaccurately and quickly survey a property when GPS does not function orlaser ranging is not possible.

RAdio Detection And Ranging (RADAR) is an RF ranging system that issometimes implemented at VHF ranges and/or UHF ranges. Different typesof RADAR systems exist, including pulsed RADAR and Continuous-Wave (CW)RADAR. Pulsed RADAR systems emit a short RF transmit pulse and listensfor an RF return. Typically, the RADAR transmitter is switched off aftertransmitting the RF transmit pulse and the RADAR receiver is switched onto listen for the RF return. However, RF ranging at shorter distancescan cause the RF return pulse to overlap the RF transmit pulse. In thiscase, the RF return pulse may be missed, since the receiver is off whentransmitting. Separate transmitters and receivers could be implementedon the same frequency, but this increases the complexity. Further,detecting the RF return pulse during the RF transmission pulse windowmay be difficult due to RF interference as the RF transmit pulse and theRF return pulse are at the same frequency.

CW RADAR systems emit a continuous RF carrier, and measure the Dopplershift of the RF return. Simple CW RADAR systems without frequencymodulation cannot determine the range to the target because there are notiming references between the continuous RF carrier and the RF return.Frequency-Modulated (FM) CW RADAR systems emit a continuous RF carrierthat is frequency modulated, and measure the frequency shift (and/or aphase shift) between the frequency modulated RF carrier and the RFreturn. The range to the target is based on the measured frequency(and/or phase) difference between the frequency modulated RF carrier andthe RF return, with the range accuracy being based on a number offactors including how accurately the system can measure the frequencydifference, how accurately the system can measure the delay time betweenthe frequency modulated RF carrier and the RF return, and the frequencyshift per unit of time used to modulate the carrier. Generally, FMCWRADAR systems that operate in VHF have an accuracy of a few meters,which is not accurate enough for survey work.

In this embodiment, RF ranging system 100 utilizes different transmitand receive frequencies, which allows RF ranging system 100 to isolatethe outgoing RF transmit signal from the return RF receive signal. In RFranging system 100, the use of a separate RF transmit and RF receivefrequencies are enabled by transponders 116-117. While typical RADARsystems operate at the same frequency in the transmit portion and thereceive portion (since the return signal in RADAR is a reflected echo ofthe transmit signal), RF ranging system 100 utilizes differentfrequencies for the transmit portion and the receive portion. Thismitigates the transmit-return overlap issue discussed above. Also, theuse of different frequencies for the transmit portion and the receiveportion mitigates issues with recovering a return signal on the samefrequency while the transmit signal is still in progress. In addition,for embodiments that utilize a lower frequency return signal than thetransmit signal, the longer wavelength of the return signal allows formore samples to be obtained, which improves the accuracy. RF rangingsystem 100 also utilizes various techniques to improve the accuracy ofranging at VHF and/or the lower frequency portions of UHF, which oftenhas a low accuracy due to the longer wavelength RF signals used. RFranging system 100 therefore is able to mitigate problems associatedwith pulse RADAR and CW RADAR at VHF.

In the embodiments described, transponders 116-117 down-convert orup-convert the RF signal that transponders 116-117 receive from rangedetector 102, and re-broadcast a new RF signal back to range detector102 at a different frequency. The use of the different RF transmit andRF receive frequencies mitigates some of the issues that are associatedwith VHF and/or UHF ranging, while enabling RF ranging system 100 tooperate in various RF-opaque environments. Another problem associatedwith VHF and/or UHF ranging is accuracy. In some embodiments, RF rangingsystem 100 uses both wavelength delay information and phase delayinformation to more accurately determine a distance to a target (e.g.,distances to transponders 116-117). This will be discussed later.

In this embodiment, range detector 102 includes a controller 104.Controller 104 includes any physical components, and/or physicalsystems, and/or physical devices that are capable of implementing thefunctionality described herein for range detector 102. While thespecific physical implementation of controller 104 is subject to designchoices, one particular embodiment may include one or more processors105 coupled with a memory 106. Processor 105 includes any electroniccircuits and/or optical circuits that are able to perform functions. Theterm “circuits” used herein refers to a physical implementation ofhardware that is capable of performing the described functionality.Processor 105 may include one or more Central Processing Units (CPU),microprocessors, Digital Signal Processors (DSPs), Application-specificIntegrated Circuits (ASICs), Programmable Logic Devices (PLDs),Field-Programmable Gate Arrays (FPGA), etc. Some examples of processorsinclude INTEL® CORE™ processors, Advanced Risk Machines (ARM®)processors, etc.

Memory 106 includes any electronic circuits and/or optical circuitsand/or magnetic circuits that are able to store data. For instance,memory 106 may store information regarding the transmitted and/or thereceived RF signal(s), which may then be used by processor 105 todetermine distances 122-123 to transponders 116-117, respectively.Memory 106 may include one or more volatile or non-volatile DynamicRandom-Access Memory (DRAM) devices, FLASH devices, volatile ornon-volatile Static RAM devices, hard drives, Solid State Disks (SSDs),shift registers, etc. Some examples of non-volatile DRAM and SRAMinclude battery-backed DRAM and battery-backed SRAM.

In some embodiments, range detector 102 may include a user interface107. User interface 107 comprises any circuits, components, or devicesthat are capable of providing information to a user 103 of rangedetector 102. For instance, user interface 107 may comprise a visualdisplay, a sound generating device, and/or a vibration generatingdevice. User interface 107 may provide various types of information touser 103, such as perpendicular distance 124 to property line 120. Forinstance, user interface 107 may visually display perpendicular distance124 to user 103, thereby allowing user 103 to determine when user 103 isproximate to property line 120. This visual representation may comprisetextual or graphical information that enables user 103 to determine arelative proximity of range detector 102 to property line, and/or whenuser 103 is at or is proximate to property line 120. In addition to orinstead of, user interface 107 may generate a sound that varies and/oris emitted when user 103 is proximate to and/or nearby property line120. For instance, a frequency of the sound may increase or decrease asrange detector 102 moves away or toward property line 120. In additionto or instead of, user interface 107 may generate a vibration thatvaries and/or is emitted when user 103 is proximate to and/or nearbyproperty line 120. For instance, an intensity, a frequency, or aparticular pattern of vibrations may be generated by user interface 107as range detector 102 moves away or toward property line 120.

In this embodiment, range detector 102 further includes an RFtransmitter 108 that is communicatively coupled with a transmit antenna110. RF transmitter 108 includes any RF circuits, and/or electroniccircuits, and/or optical circuits that are capable of transmitting RFsignals. For instance, RF transmitter 108 may be capable of generatingand modulating a carrier frequency in the VHF range (e.g., a carrierfrequency around 151.5 MHz) and/or portions of the UHF range (e.g., acarrier frequency in the 70 cm band or the 23 cm band). Transmit antenna110 typically is designed with a particular frequency or frequency rangein mind. For instance, transmit antenna 110 may be designed to operateat or around 151.5 MHz, the 70 cm band, and/or the 23 cm band dependingon the frequency and/or frequency range that RF transmitter 108 iscapable of utilizing.

Range detector 102 in this embodiment also includes an RF receiver 112that is communicatively coupled with a receive antenna 114. RF receiver112 includes any RF circuits, and/or electronic circuits, and/or opticalcircuits that are capable of receiving RF signals. For instance, RFreceiver 112 may be used to receive and/or demodulate a carrierfrequency in the VHF range (e.g., a carrier frequency around 75.75 MHz)or in a lower frequency portion of the UHF range (e.g., a carrierfrequency in the 70 cm or 23 cm band).

While the specific elements illustrated for range detector 102 have beenshown in this embodiment as separate elements, other embodiments maycombine the elements to achieve the same functionality. For instance,controller 104 may directly implement the functionality described hereinfor RF transmitter 108 and/or RF receiver 112.

Consider that RF ranging system 100 is in operation and that rangedetector 102 is located proximate to property line 120. FIG. 2 is a flowchart of a method 200 of determining a perpendicular distance to aproperty line in an illustrative embodiment. The methods describedherein will be discussed with respect to RF ranging system 100, althoughthe methods may be performed by other systems, not shown. The stepsillustrated for the methods described herein may be performed in analternate order. Also, the methods described herein may include othersteps, not shown. Further, although specific examples are provided belowwith respect to RF activities in the UHF band, the performance of method200 may utilize other bands as desired (e.g., the 70 cm or 23 cm band).

To begin a process to calculate distance 122 to transponder 116,processor 105 directs RF transmitter 108 to broadcast an RF signal 128at a first carrier frequency (f1), in step 202. For example, processor105 may generate a Barker code, and direct RF transmitter 108 tomodulate a 151.5 MHz carrier (or a carrier in the 70 cm or 23 cm band)with the Barker code to broadcast RF signal 128. A Barker code (orBarker sequence) is a finite sequence of N values of +1 and −1 whichhave exceptional autocorrelation properties. Autocorrelation is thecorrelation of a signal with a delayed copy of itself as a function ofdelay. Autocorrelation is used by pulse RADAR, since the reflected RFreturn signal off of the target is a delayed copy of the RF transmittedsignal. In RADAR, an autocorrelation is performed between thetransmitted RF pulse signal and the reflected RF return signal todetermine the delay between the transmitted RF pulse signal and thereflected RF return signal. The delay is used to calculate a distance tothe target based on the speed of light through the transmission medium(e.g., through the atmosphere).

Cross-correlation is a measure of the similarity of two functions as afunction of displacement of one relative to another. This is also knownas a sliding dot product or sliding inner-product. Cross-correlation issimilar to autocorrelation, with the difference being that the twosignals that are cross-correlated are different signals.Cross-correlation is used by RF ranging system 100, since the targets(e.g., transponders 116-117) provide a delayed copy of the transmittedsignal at a different frequency.

As discussed previously, Barker codes have excellent autocorrelationproperties. Currently, only nine Barker codes are known. The shortestlength N Barker code is two, and the longest length N is thirteen.Barker codes of length N equal to eleven and thirteen are used indirect-sequence spread spectrum and pulse compression radar systemsbecause of their autocorrelation properties (e.g., the sidelobe level ofamplitude of the Barker codes is 1/N compared to the peak signal). Forinstance, the peak-to-sidelobe ratio for an 11-bit Barker code is −20.8dB, and the sidelobes have an equal magnitude.

In response to broadcasting RF signal 128 at f1, one of transponders116-117 responds to RF signal 128. In one embodiment, RF signal 128includes an address that indicates which of transponders 116-117 willrespond to RF signal 128. This precludes both of transponders 116-117from responding at the same time. However, other methods exist topreclude or mitigate the effects of having both of transponders 116-117responding at the same time. For instance, each of transponders 116-117may be configured to respond on different frequencies.

For purposes of discussion, assume that transponder 116 responds to RFsignal 128. For instance, RF signal 128 may include an address that isassociated with transponder 116. In response to transponder 116receiving RF signal 128 from range detector 102, transponder 116transmits an RF rebroadcast 132 of RF signal 128 on a second carrierfrequency (f2) that is different from f1. For instance, transponder 116may receive RF signal 128 from range detector 102, down-convert RFsignal 128 from 151.5 MHz to a 75.75 MHz signal, and transmit arebroadcast of RF signal 128 at 75.75 MHz. Down-converting may beperformed in the analog domain and/or in the digital domain. Forexample, transponder 116 may utilize a digital downconverter to directlyconvert RF signal 128 at 151.5 MHz to 75.75 MHz, prior to transmittingRF rebroadcast 132. In addition to or instead of, transponder 116 maydirectly sample RF signal 128 to recover a datastream encoded by RFsignal 128, and modulate an RF carrier at f2 using the datastream or ascaled version of the datastream. In some embodiments, RF signal 128 ismodulated using a Barker code. The modulation may include amplitudemodulation, frequency modulation, phase modulation, or some combinationof amplitude modulation, frequency modulation, and phase modulation.

In response to the rebroadcast by transponder 116, processor 105 ofrange detector 102 receives RF rebroadcast 132 (e.g., via RF receiver112), in step 204. For instance, RF receiver 112 may receive RFrebroadcast 132 at f2 (via receive antenna 114), and provide informationregarding RF rebroadcast 132 to processor 105.

Processor 105 calculates distance 122 to transponder 116 based on RFsignal 128 and RF rebroadcast 132, in step 206. Distance 122 may becalculated in a number of ways. For example, a cross-correlation may beperformed between RF signal 128 and RF rebroadcast 132, which mayprovide information about distance 122. Phase differences between RFsignal 128 and RF rebroadcast 132 may also provide information aboutdistance 122. Timing information regarding when RF signal 128 istransmitted by range detector 102 and when RF rebroadcast 132 isreceived by range detector 102 may also provide information aboutdistance 122. Various mechanisms exist, and the previous examples arenot to be considered as all-inclusive. Further, performing across-correlation process between RF signal 128 and RF rebroadcast 132may include frequency scaling that normalizes RF signal 128 and/or thedatastream used to modulate RF signal 128 with respect to RF rebroadcast132. For example, if f2=f1/2, then processor 105 may down-convert thedatastream used to modulate RF signal 128 by two prior to performing across-correlation with the datastream that modulates RF rebroadcast 132.

To begin a process to calculate distance 123, processor 105 directs RFtransmitter 108 to broadcast an RF signal 130 at f1, in step 208. Forexample, processor 105 may generate a Barker code, and direct RFtransmitter 108 to modulate a 151.5 MHz carrier with the Barker code tobroadcast RF signal 130.

In response to broadcasting RF signal 130 at f1, one of transponders116-117 responds to RF signal 130. For purposes of discussion, assumethat transponder 117 responds to RF signal 130. For example, RF signal130 may include an address associated with transponder 117. In responseto transponder 117 receiving RF signal 130 from range detector 102,transponder 117 transmits an RF rebroadcast 134 of RF signal 130 at f2.For instance, transponder 117 may receive RF signal 130 from rangedetector 102, down-convert RF signal 130 broadcast by range detector 102at 151.5 MHz to a 75.75 MHz signal, and transmit a rebroadcast of RFsignal 130 at 75.75 MHz. Down-converting may be performed in the analogdomain and/or in the digital domain. For example, transponder 116 mayutilize a digital downconverter to directly convert RF signal 128 at151.5 MHz to 75.75 MHz, prior to transmitting RF rebroadcast 132. Inaddition to, or instead of, transponder 117 may directly sample RFsignal 130 to recover a datastream encoded by RF signal 130, andmodulate an RF carrier at f2 using the datastream or a scaled version ofthe datastream. In some embodiments, RF signal 130 is modulated using aBarker code. The modulation may include amplitude modulation, frequencymodulation, phase modulation, or some combination of amplitudemodulation, frequency modulation, and phase modulation.

In response to the rebroadcast by transponder 117, processor 105 ofrange detector 102 receives RF rebroadcast 134 (e.g., via RF receiver112), in step 210. For instance, RF receiver 112 may receive RFrebroadcast 134 at f2 (via receive antenna 114), and provide informationregarding RF rebroadcast 134 to processor 105.

Processor 105 calculates distance 123 to transponder 117 based on RFsignal 130 and RF rebroadcast 134, in step 212. Distance 123 may becalculated in a number of ways. For example, a cross-correlation may beperformed between RF signal 130 and RF rebroadcast 134, which mayprovide information about distance 123. Phase differences between RFsignal 130 and RF rebroadcast 134 may also provide information aboutdistance 123. Timing information regarding when RF signal 130 istransmitted by range detector 102 and when RF rebroadcast 134 isreceived by range detector 102 may also provide information aboutdistance 123. Various mechanisms exist, and the previous examples arenot to be considered as all-inclusive. Further, performing across-correlation process between RF signal 130 and RF rebroadcast 134may include frequency scaling that normalizes RF signal 130 and/or thedatastream used to modulate RF signal 130 with respect to RF rebroadcast134. For example, if f2=f1/2, then processor 105 may down-convert thedatastream used to modulate RF signal 130 by two prior to performing across-correlation with the datastream that modulates RF rebroadcast 134

In response to calculating distance 122 and distance 123, processor 105calculates perpendicular distance 124 to property line 120 based ondistance 122, distance 123, and known distance 126, in step 214. Thiscan be solved geometrically. For example, distance 122 squared is equalto perpendicular distance 124 squared plus distance 137 squared. Also,distance 123 squared is equal to perpendicular distance 124 squared plusdistance 136 squared. Distance 122 and distance 123 have beencalculated, and distance 126 is known. Since distance 126 is known,there is a relationship between distance 136 and distance 137. Theserelationships and the known and calculated values can be used tocalculate perpendicular distance 124 by solving a system that has threesimultaneous equations and three unknown variables.

FIG. 3 illustrates additional steps of method 200 in an illustrativeembodiment. In particular, the steps of FIG. 3 describe how an integerwavelength delay and a fractional phase delay may be calculated, whichis one possible mechanism for calculating distances 122-123. Asdescribed herein, an integer wavelength delay is an integer number ofwavelengths between either transponder 116 and range detector 102 (fordistance 122), or transponder 117 and range detector 102 (for distance123). Since two different wavelengths are used by RF ranging system 100,either can be used to represent the integer wavelength delay. There isno particular advantage of the choice of one over the other. Rather, itis a design choice. Choosing one wavelength over the other merelyentails modifying either RF signals or the RF responses based on arelationship between f1 and f2 (e.g., using a scaling factor tonormalize RF signals and RF responses with respect to each other).

To begin a process to calculate distance 122 between transponder 116 andrange detector 102, processor 105 performs a cross-correlation betweenRF signal 128 and RF rebroadcast 132, in step 302. For example, if RFsignal 128 and RF rebroadcast 132 are sampled at frequency f_(s), thenthe cross-correlation between the samples of RF signal 128 and thesamples of RF rebroadcast 132 generates a data sequence that can beanalyzed. The data sequence will have a peak that identifies the lag ofRF rebroadcast 132 with respect to RF signal 128. This lag is quantized,and is an integer unit of the sample period 1/f_(s). As discussedpreviously, RF signal 128 may be normalized with respect to RFrebroadcast 132 based on the relationship between f1 and f2. In someembodiments, RF signal 128 and/or RF rebroadcast 132 may be converted toan intermediate frequency prior to performing a cross-correlation.

Processor 105 calculates an integer wavelength delay based on the firstcorrelation, in step 304. For example, since the speed of light in theatmosphere (c_(atm)) is known, a rough estimate of the round tripdistance (d_(r)) between range detector 102 and transponder 116 islag×(c_(atm)/f_(s).)−(Δd×c_(atm)), where Δd is a delay time between whentransponder 116 receives RF signal 128 and when transponder 116transmits RF rebroadcast 132. Δd can be measured. Knowing the round tripdistance d_(r), an estimate of distance 122 between transponder 116 andrange detector 102 is d_(r)/2+/−(c_(atm)/f_(s)).

FIG. 4 illustrates an estimated distance 401 between range detector 102and transponder 116 in an illustrative embodiment. Estimated distance401 has an accuracy of +/−(c_(atm)/f_(s)). Typically this accuracy willnot be sufficient by itself for a survey. For instance, the accuracy maybe about +/−1 meter. However, +/−1 meter is sufficient to calculate theinteger wavelength delay for distance 122 in terms of either f1 or f2,since c_(atm), f1, f2, are known, and d_(r)/2+/−(c_(atm)/f_(s)) can becalculated. For instance, the integer wavelengths 402-405 of f2 areillustrated in FIG. 4. Estimated distance 401 is illustrated as withininteger wavelength 405 (N=4). Although estimated distance 401 has anerror term, distances 402-404 are known, and are based on either f1 orf2. In the example, processor 105 would calculate the integer wavelengthdelay as three wavelengths, corresponding to distance 402+distance403+distance 404. If f2 is used as a reference in the integer wavelengthdelay, then the distance represented by a 3 wavelength delay wouldcorrespond to a distance of 3×(c_(atm)/f2). If f1 is used as areference, the distance represented by a three wavelength delay wouldcorrespond to a distance of 3×(c_(atm)/f1). Generally, the integerwavelength delay is the magnitude of (estimated distance401)/(c_(atm)/f1). or the magnitude of (estimated distance401)/(c_(atm)/f2), depending on whether f1 or f2 is used as a reference.

Next, processor 105 calculates a phase difference between RF signal 128and RF rebroadcast 132, in step 306. The phase difference can be used tocalculate a fractional wavelength delay. FIG. 5 illustrates an exampleof a phase difference 502 between RF signal 128 and RF rebroadcast 132in an illustrative embodiment. A fractional wavelength delay is one ofthe terms in a distance calculation (in addition to the integerwavelength delay) that is based on the phase difference between thecarrier (f1) of RF signal 128 and the carrier (f2) of RF rebroadcast132. Since the wavelengths of f1 and f2 are known, the fractionalwavelength delay provides information regarding the sub-wavelengthdistance 406 (see FIG. 4) between point A and range detector 102. Eachdegree of phase shift corresponds to a distance of 1/360×(c_(atm)/f2),if f2 is used as a reference, or 1/360×(c_(atm)/f1) if f1 is used as areference. Using information regarding f1, f2, and phase difference 502between RF signal 128 and RF rebroadcast 132, processor 105 calculatesthe fractional wavelength delay, in step 308. The fractional wavelengthdelay is ((phase difference 502)/360)×1/f2, if f2 is used as reference,and ((phase difference 502)/360)×1/f1, if f1 is used as a reference. Insome embodiments, RF signal 128 and/or RF rebroadcast 132 may beconverted to an intermediate frequency prior to determining phasedifference 502.

In response to calculating the integer wavelength delay (int_(d)) andthe fractional wavelength delay, processor 105 calculates distance 122between range detector 102 and transponder 116, in step 310. If f2 isused as a reference, then distance 122 would be ((int_(d))+((phasedifference 502)/360))×(c_(atm)/f2). If f1 is used as a reference, thedistance 122 would be ((int_(d))+((phase difference502)/360))×(c_(atm)/f1).

For instance, if f2 is 75.75 MHz, int_(d) is three (with respect to f2),phase difference 502 is 10 degrees, and c_(atm) is 2.997×10⁸ m/s, thendistance 122 (reference to f2) would be (3+10/360)×(2.997×10⁸m/s)/75.75×10⁶, or 11.9792 meters.

Distance 123 can be calculated in a similar manner as distance 122, byperforming steps 312-320 with respect to RF signal 130 and RFrebroadcast 134 to calculate distance 123 between range detector 102 andtransponder 117. In particular, processor 105 performs cross-correlationbetween RF signal 130 and RF rebroadcast 134 in step 312, which may besimilar to the process described in step 304. In some embodiments, RFsignal 130 and/or RF rebroadcast 134 may be converted to an intermediatefrequency prior to performing a cross-correlation.

Processor 105 may then calculate an integer wavelength delay based onthe cross-correlation, in step 314, which may be similar to the processdescribed in step 306. Processor 105 may further determine a phasedifference between RF signal 130 and RF rebroadcast 134, in step 316,which may be similar to the process described in step 308. In someembodiments, RF signal 130 and/or RF rebroadcast 134 may be converted toan intermediate frequency prior to determining the phase difference.Using the phase difference, processor 105 calculates the fractionalwavelength delay based on the phase difference, in step 318. Step 318may be similar to the process described for step 308. Processor 105 maythen calculate distance 123 based on the integer wavelength delay andthe fractional wavelength delay calculated for RF signal 130 and RFrebroadcast 134, in step 320. Step 320 may be similar to the processdescribed for step 310 above. Using values calculated for distances122-123, processor 105 calculates perpendicular distance 124 using thePythagorean theorem based on the relationships between distances 122-123and known distance 126, in step 214.

RF ranging system 100 utilizes range detector 102 in coordination withtransponders 116-117 to determine perpendicular distance 124 using RFranging. This allows for a surveyor to efficiently and quickly identifya boundary of a property, such as illustrated at property line 120. Whenrange detector 102 is in motion, range detector 102 may provideinformation regarding changes to perpendicular distance 124 as distances122-123 change in real-time or near real-time. For example, rangedetector 102 may generate an alert when range detector 102 determinesthat perpendicular distance 124 is zero or approximately zero, whichindicates to a surveyor that range detector 102 is on property line 120.

Example

The following example illustrates one possible implementation of some ofthe functionality described herein for RF ranging system 100. Theexample is not intended to limit the scope of the claims nor representeither a preferred embodiment or the only embodiment to implement thefunctionality described herein. As such, one of ordinary skill in theart will recognize that RF ranging system 100 may be implemented indifferent ways as a matter of design choice. In particular, the examplewill be described with respect to FIG. 6, which is a block diagram ofranging system 100 in another illustrative embodiment. In thisembodiment, range detector 602 includes a controller 604. Controller 604includes a first shift register 605, a second shift register 606, and aphase detector 607.

Range detector 602 sends a pulse sequence 628 consisting of a VHFcarrier directly modulated by Barker codes. The use of a signal with awavelength on the same order of magnitude or greater than the diameterof objects such as tree trunks or leaves, allow the radio signals topropagate through foliage with less multipath and attenuation than withhigher frequencies commonly used for RADAR, for example. Leaves andtrees generally have diameters of less than one meter. Therefore, theuse of wavelengths of greater than 1 meter is advantageous fortransmission and reception in a jungle environment.

Transponder 616 receives the modulated portion of the pulse sequence 628using a threshold, and utilizes high speed circuitry to perform directconversion. Transponder 616 divides the received sequence timing by twousing high speed logic. The divided signals, which are at ½ the originalfrequency, are transmitted back to range detector 602 as RF rebroadcast632. Transponder 616 utilizes a PLD that includes a high-speeddeterministic data flow path to reduce latency and jitter in directconversion.

Range detector 602 and transponders 616-617 utilize separate transmitand receive antennas for the original carrier and the ½ frequencysignal. The separate antennas are orthogonally polarized to allow forfull duplex operation. In addition, the separate antennas are resonateand selective for the frequency intended. Finally, the antennas consistof multiple elements to form a beam and provide directivity (e.g., moregain in the desired direction).

Range detector 602 receives the ½ frequency signal from transponder 616,which is directly sampled from RF rebroadcast 632 and captured in firstshift register 605. Range detector 602 also simultaneously directlysamples pulse sequence 628, which is captured in second shift register606. The directly sampled signal from transponder 616 is also sent to afirst input of a phase detector 607 consisting of an exclusive NOR gate.The transmit modulation Barker code carrier of range detector 602 isdivided by two and is also sent to a second input of phase detector 607.Range detector 602 measures the time delay from its transmitted sequenceto the received sequence two ways. First, phase detector 607 produces apulse width modulated logic signal which is low pass filtered to removethe carrier frequency and produce a DC voltage proportional to the phasedifference of transponder 616 and range detector 602 divided by twosignals. The phase voltage is captured by an analog to digital converter(ADC, not shown). The ADC value is normalized to become the fractionalwavelength delay (k). Further, first shift register 605 and second shiftregister 606 are auto-correlated mathematically. The correlationsequence exhibits a peak at a particular time. This can be used todetermine the integral number of wavelength delays (n).

The total time delay then becomes nT+Kt, where T represents the period.The time delay is longer than the speed of light multiplied by the roundtrip distance. This is due to an offset caused by delays in theelectronics of range detector 602 and transponder 616, and also by theindex of refraction of the atmosphere. These two effects are linear andcan be calibrated out by taking measurements at known distances. usingd=mt+b, where d is the distance, t is the time delay measured, and b isthe delay offset. Using the total time delay and the offset, rangedetector 602 can calculate distance 622 between transponder 616 andrange detector 602. This process is repeated for transponder 617 tomeasure distance 623 based on pulse sequence 629 and RF rebroadcast 633.With distances 622-623 measured, range detector 602 can calculateperpendicular distance 624.

FIG. 7 is a block diagram of RF ranging system 100 in anotherillustrative embodiment. In this embodiment, transponders 116-117 aremounted to aerial vehicles 702-703, respectively, which are locatedabove and offset from known coordinates 706-707 of property line 120.For example, known coordinates 706-707 may comprise the corners ofproperty line 120. Known coordinates 706-707 may be represented in a 3-Dcoordinate system. In some embodiments, aerial vehicles 702-703 compriseUnmanned Aerial Vehicles (UAVs), which may operate autonomously, underthe direction of range detector 102, under the direction of user 103, orsome combination thereof.

Although two aerial vehicles 702-703 are depicted in FIG. 7, RF rangingsystem 100 may utilize more or fewer aerial vehicles as a matter ofdesign choice. For example, RF ranging system 100 may utilize one aerialvehicle (e.g., aerial vehicle 702) to perform the functionality of bothaerial vehicles 702-703 by traveling between areas proximate to knowncoordinates 706-707 of property line 120 during a survey.

In this embodiment, aerial vehicles 702-703 include a Global PositioningSystem (GPS) receivers 704-705, respectively, which communicate with GPSsatellites in orbit (not shown) to accurately provide location data toaerial vehicles 702-703. For example, aerial vehicles 702-703 may hoverabove foliage proximate to known coordinates 706-707 of property line120, which allows aerial vehicles 702-703 to receive GPS signals andcalculate their location in the 3-D coordinate system. In someembodiments, aerial vehicles 702-703 include additional sensors (notshown) that allow aerial vehicles 702-703 to calculate their respectiveheights 710-711 above the ground 712 with a higher degree of accuracythan may be derived from the GPS location data alone.

Typically, heavy foliage during a survey can render GPS receivers on theground 712 unusable, but aerial vehicles 702-703 are capable ofoperating above the foliage in RF ranging system 100, thereby operatingabove the RF-opaque environment. Further, as the coordinates of knowncoordinates 706-707 of property line 120 are known in the 3-D coordinatesystem, distances 708-709 between aerial vehicles 702-703 and knowncoordinates 706-707, respectively, can be calculated based on the GPSderived 3-D location data for aerial vehicles 702-703 and knowncoordinates 706-707 of property line 120.

Transponders 116-117 are separated from each other by a computeddistance 126, which may be calculated from the 3-D location data ofaerial vehicles 702-703 derived from their GPS receivers 704-705. Aerialvehicle 702 is separated from range detector 102 by distance 122, whichis measured by range detector 102 as previously described in priorembodiments. Aerial vehicle 703 is separated from range detector 102 bydistance 123, which is also measured by range detector 102 as previouslydescribed in prior embodiments. Range detector 102 calculatesperpendicular distance 124 to property line 120 based on thetrigonometric and/or 3-D location data of aerial vehicles 702-703 (e.g.,location one (x1, y1, z1) of aerial vehicle 702 and location two (x2,y2, z2) of aerial vehicle 703), known coordinates 706-707, distances708-709, distances 136-137, and/or heights 710-711 of aerial vehicles702-703 above the ground 712.

FIG. 8 is a block diagram of RF ranging system 100 in anotherillustrative embodiment. In this embodiment, aerial vehicle 702 is usedto calculate the location of range detector 102, (e.g., location Dcorresponding to coordinates x4, y4, z4 in the 3-D coordinate system)which is currently unknown. For example, range detector 102 may be inuse on the ground 712 in dense foliage, which may preclude the use ofGPS to determine the location of range detector 102 (and user 103). Inthis embodiment, distances d1-d3 between range detector 102 and aerialvehicle 702 are calculated by range detector 102 in response to aerialvehicle 702 moving between different locations A-C (e.g., above thefoliage). The calculated distances d1-d3 and the coordinates of thedifferent locations A-C generated via GPS receiver 704 are then used byrange detector 102 to determine the coordinates at location D of rangedetector 102 in the 3-D coordinate system.

Aerial vehicle 702 begins at or moves to or proximate to location A, anddetermines its 3-D coordinates (x1, y1, z1) using GPS receiver 704. Asdiscussed previously with respect to FIG. 7, aerial vehicle 702 maycalculate height 710 above the ground 712 in order to improve theaccuracy of the GPS information used to determine its 3-D coordinates(x1, y1, z1). Range detector 102 calculates distance 802 (d1) to aerialvehicle 702 using the techniques described for the previous embodiments(e.g., any of the previously described steps for calculating distances122-123).

Aerial vehicle 702 then moves via path 804 to location B, and determinesits coordinates (x2, y2, z2) using GPS receiver 704. Height 710 may alsobe used to improve the accuracy of the GPS information used to determineits 3-D coordinates (x2, y2, z2). In this embodiment, location B isdifferent than location A. Range detector 102 calculates distance 806(d2) to aerial vehicle 702 using the techniques described for theprevious embodiments (e.g., any of the previously described steps forcalculating distances 122-123).

Aerial vehicle 702 then moves via path 808 to location C, and determinesits coordinates (x3, y3, z3) using GPS receiver 704. Height 710 may alsobe used to improve the accuracy of the GPS information used to determineits 3-D coordinates (x3, y3, z3). In this embodiment, location C isdifferent than either location A or location B. Range detector 102calculates distance 810 (d3) to aerial vehicle 702 using the techniquesdescribed for the previous embodiments (e.g., any of the previouslydescribed steps for calculating distances 122-123).

Distance 802 (d1), distance 806 (d2) and distance 810 (d3) arerepresented by the following formulas:

d ₁ ²=(x ₁ −x ₄)+(y ₁ −y ₄)²+(z ₁ −z ₄)²

d ₂ ²=(x ₂ −x ₄)²+(y ₂ −y ₄)²+(z ₂ −z ₄)²

d ₃ ²=(x ₃ −x ₄)²+(y ₁ −y ₄)²+(z ₃ −z ₄)²

Solving for (x4, y4, z4), results in three systems of three equationsand 3 unknowns:

System one:

x ₄ =x ₁ ±{d ₁ ²−(y ₁ −y ₄)²−(z ₁ −z ₄)²}^(1/2)

x ₄ =x ₂ ±{d ₂ ²−(y ₂ −y ₄)²−(z ₂ −z ₄)²}^(1/2)

x ₄ =x ₃ ±{d ₃ ²−(y ₁ −y ₄)²−(z ₃ −z ₄)²}^(1/2)

System two:

y ₄ =y ₁ ±{d ₁ ²−(x ₁ −x ₄)²−(z ₁ −z ₄)²}^(1/2)

y ₄ =y ₂ ±{d ₂ ²−(x ₂ −x ₄)²−(z ₂ −z ₄)²}^(1/2)

y ₄ =y ₃ ±{d ₃ ²−(x ₃ −x ₄)²−(z ₃ −z ₄)²}^(1/2)

System three:

z ₄ =z ₁ ±{d ₁ ²−(y ₁ −y ₄)²−(x ₁ −x ₄)²}^(1/2)

z ₄ =z ₂ ±{d ₂ ²−(y ₂ −y ₄)²−(x ₂ −x ₄)²}^(1/2)

z ₄ =z ₃ ±{d ₃ ²−(y ₃ −y ₄)²−(x ₃ −x ₄)²}^(1/2)

The equations for system one, system two, and system three will yieldtwo solutions due to the +/− terms. The solution which is common to allcases is the correct one.

An example is shown below in table below with the correct solutionunderlined:

TABLE 1 Dist. to coordinate coordinate coordinate range position Devicex y z detector 102 x1, y1, z1 Aerial −11  −3  22 28.07134 vehicle 702x2, y2, z2 Aerial −11  0 22 27.36786 vehicle 702 x3, y3, z3 Aerial 13 929 27.94638 vehicle 702 x4, y4, z4 Range x₄ y₄ z₄ Detector 102 +solution − solution solution x4  7 −29  pos x1, y4  5 −11  y1, z1 z4 422 solution x4  7 −29  pos x2, y4  5 −5  y2, z2 z4 42 2 — solution x4 197 pos x3, y4 13 5 y3, z3 z4 56 2

Based on the example above, the location of range detector 102 is (7,5,2) in the coordinate system. In this example, the GPS coordinates ofthe transponders are translated to the East, North, Up (ENU) coordinatesystem. The location of the range detector in 3D space is then foundusing “true range multilateration” mathematics. The technique uses themultiple ranges (distances) between the range detector and thespatially-separated transponders at known locations to solve for thecoordinates of the range detector. The ENU solution is then translatedback into the GPS coordinate system to generate the resultingcoordinates for range detector 102.

Although FIG. 8 has been described with respect to one aerial vehiclethat moves, this embodiment may utilize two or more aerial vehicle asdesired. For example, aerial vehicles 702-703 may be utilized, with oneof aerial vehicles 702-703 moving to two of locations A-C during thecalculations to determine the coordinates (x4, y4, z4) at location D ofranged detector 102, while the other aerial vehicle remains proximate toone of locations A-C. In another example, three aerial vehicle may beused, each of which may be proximate to one of locations A-C todetermine the location of range detector 102.

FIG. 9 is a flow chart of a method 900 of determining a perpendiculardistance to a property line in an illustrative embodiment. Method 900will be discussed with respect to RF ranging system 100 of FIG. 7,although method 900 may be performed by other systems, not shown.

First, aerial vehicle 702 begins at or moves to location one proximateto known coordinates 706 of property line 120 (see step 902). Forexample, known coordinates 706 may be a corner of property line 120.Range detector 102 determines the coordinates (x1, y1, z1) of aerialvehicle 702 at location one (see step 904). For example, aerial vehicle702 utilizes GPS receiver 704 to determine its coordinates (x1, y1, z1)in a 3-D coordinate system, and provides this information to rangedetector 102. As discussed previously, height 710 of aerial vehicle 702above the ground 712 may be used to improve the accuracy of the GPSinformation used by aerial vehicle 702 to calculate its coordinates (x1,y1, z1).

Range detector 102 broadcasts RF signal 128 (e.g., with f1 at 427.5 MHz,see step 906), which may be the same or similar to step 202, previouslydescribed. Range detector 102 receives RF rebroadcast 132 (e.g., with f2at 442.5 MHz), from transponder 116 on aerial vehicle 702 (see step908). Step 908 may be similar to step 204, previously described. Rangedetector 102 calculates distance 122 to aerial vehicle 702 (see step910), which may be similar to step 206, previously described. Forexample, step 910 may include some or all of the previously describedsteps 301-310 (see FIG. 3) for calculating integer wavelength delays andfractional wavelength delays.

Next, aerial vehicle 703 moves to location two proximate to knowncoordinates 707 of property line 120 (see step 912). For example, knowncoordinates 707 may be a different corner of property line 120. Rangedetector 102 determines the coordinates (x2, y2, z2) of aerial vehicle703 at location two (see step 914). For example, aerial vehicle 703utilizes GPS receiver 705 to determine its coordinates (x2, y2, z2) inthe 3-D coordinate system, and provides this information to rangedetector 102. As discussed previously, height 711 of aerial vehicle 703above the ground 712 may be used to improve the accuracy of the GPSinformation used to calculate its coordinates (x2, y2, z2).

Range detector 102 broadcasts RF signal 130 (e.g., with f1 at 427.5 MHz,see step 916), which may be the same or similar to step 208, previouslydescribed. Range detector 102 receives RF rebroadcast 134 (e.g., with f2at 442.5 MHz), from transponder 117 on aerial vehicle 703 (see step918). Step 918 may be similar to step 210, previously described. Rangedetector 102 calculates distance 123 to aerial vehicle 703 (see step920), which may be similar to step 212, previously described. Forexample, step 920 may include some or all of the previously describedsteps 312-320 (see FIG. 3) for calculating integer wavelength delays andfractional wavelength delays.

Range detector 102 calculates a perpendicular distance 124 to propertyline 120 (see step 922). For example, range detector 102 may utilizedistances 122-123, known coordinates 706-707, the GPS derivedcoordinates (x1, y1, z1; x2, y2, z2) of aerial vehicles 702-703,distances 708-709, distance 126, heights 710-711, or other informationto calculate perpendicular distance 124 to property line 120.

Although method 900 has been described with respect to the use of twoaerial vehicles 702-703, one aerial vehicle (e.g., aerial vehicle 702)may be used to perform the functions of both aerial vehicles 702-703.For example, aerial vehicle 702 may travel from location one to locationtwo in order for steps 912-922 to be repetitively performed.

FIG. 10 is a flow chart of a method 1000 of determining the coordinatesof a range detector in an illustrative embodiment. Method 1000 will bediscussed with respect to RF ranging system 100 of FIG. 8, althoughmethod 1000 may be performed by other systems, not shown. Step 1002comprises initially moving aerial vehicle 702 proximate to location A.Proximate to location A, aerial vehicle 702 determines its 3-Dcoordinates (x1, y1, z1) using GPS receiver 704, which may be augmentedby calculating height 710 of aerial vehicle 702 above the ground 712(see step 1004). While aerial vehicle 702 is proximate to location A,range detector 102 broadcasts RF signal 128-1 (e.g., with f1 at 427 MHz,see step 1006), which may be similar to step 202 previously described.Range detector receives RF rebroadcast 132-1 from transponder 116 (e.g.,with f2 at 442.5 MHz, see step 1008). Step 1008 may be similar to step204, previously described. Range detector 102 calculates distance d1based on RF signal 128-1 and RF rebroadcast 132-1 (see step 1010). Forexample, step 1010 may include some or all of the previously describedsteps 301-310 (see FIG. 3) for calculating integer wavelength delays andfractional wavelength delays as part of the distance calculation.

In step 1012, a determination is made if aerial vehicle 702 is to moveto additional locations. As only location A and distance d1 has beencalculated so far, step 1014 is performed to change to the next locationB. Aerial vehicle 702 moves proximate to location B via path 804, andsteps 1004-1010 are repeated using RF broadcast 128-2, RF rebroadcast132-2, coordinates x2, y2, z2) to determine distance d2. In thisembodiment, location C and distance d3 remain, so step 1014 is performedto change to the next location B. Aerial vehicle 702 moves proximate tolocation C via path 808, and steps 1004-1010 are repeated using RFbroadcast 128-3, RF rebroadcast 13232, coordinates x3, y3, z3) todetermine distance d3.

If no more locations remain for analysis, range detector 102 determinesits coordinates x4, y4, z4 at location D using d1, d2, and d3 (see step1016). In some embodiments, more or fewer locations may be analyzed byperforming steps 1004-1010 when determining the coordinates x4, y4, z4at location D.

Although method 1000 has been described using one aerial vehicle 702,additional aerial vehicles may be used in other embodiments, which wouldpreclude some or all of the location changes previously described foraerial vehicle 702 (e.g., three aerial vehicles may be used, oneproximate to each of locations A-C). Further, the aerial vehiclesdescribed herein may operate autonomously, under direction of rangedetector 102, and/or under direction of user 103 in differentembodiments.

Although various RF techniques may be employed by RF ranging system 100in order to calculate distances from range detector 102 to transponders116-117, and aerial vehicles 702-703, when they include transponders116-117, specialized techniques may be used when f1 and f2 are separatedin frequency by a small amount (e.g., a few MHz). In this case, phaselocked frequency mixing can be employed. For example, range detector 102may utilize a frequency mixer to shift its original base-band signal tof1, which is the transmit frequency of range detector 102. Next,transponders 116-117 utilize a phase locked homodyne method to firstshift their received frequency f1 to base-band, and then utilize afrequency mixer to shift the base-band signal up to the transmitfrequency of transponders 116-117, which is f2. Range detector 102utilizes a phase locked homodyne method to shift its received frequencyf2 to base-band to compute the coarse delay nT relative to its originalbase-band signal. The phase lock circuitry also provides the necessaryinformation for phase comparison of the transmit and received carrierfrequencies used by range detector 102 to determine the fine delay Kt.

The use of aerial vehicle that operate above RF-opaque environmentsenables range detector 102 to calculate its location and/or to calculatea distance and/or vector from range detector 102 to another 3-Dcoordinate in the surrounding environment, such as a property line or apoint in the 3-D coordinate system. These activities provide a technicalbenefit of enabling survey and/or location capabilities in environmentsthat preclude the use of GPS and/or laser ranging, such as a jungle,thereby improving the art.

Any of the various elements shown in the figures or described herein maybe implemented as hardware, software, firmware, or some combination ofthese. For example, an element may be implemented as dedicated hardware.Dedicated hardware elements may be referred to as “processors”,“controllers”, or some similar terminology. When provided by aprocessor, the functions may be provided by a single dedicatedprocessor, by a single shared processor, or by a plurality of individualprocessors, some of which may be shared. Moreover, explicit use of theterm “processor” or “controller” should not be construed to referexclusively to hardware capable of executing software, and mayimplicitly include, without limitation, digital signal processor (DSP)hardware, a network processor, application specific integrated circuit(ASIC) or other circuitry, field programmable gate array (FPGA), readonly memory (ROM) for storing software, random access memory (RAM),non-volatile storage, logic, or some other physical hardware componentor module.

Also, an element may be implemented as instructions executable by aprocessor or a computer to perform the functions of the element. Someexamples of instructions are software, program code, and firmware. Theinstructions are operational when executed by the processor to directthe processor to perform the functions of the element. The instructionsmay be stored on storage devices that are readable by the processor.Some examples of the storage devices are digital or solid-statememories, magnetic storage media such as a magnetic disks and magnetictapes, hard drives, or optically readable digital data storage media.

Although specific embodiments were described herein, the scope of theinvention is not limited to those specific embodiments. The scope of theinvention is defined by the following claims and any equivalentsthereof.

What is claimed is:
 1. An apparatus comprising: a range detectorconfigured to transmit Radio Frequency (RF) signals at a first carrierfrequency (f1), and to receive RF signals at a second carrier frequency(f2) that is different than f1, wherein f1 and f2 are selected fromfrequencies of 30 Megahertz (MHz) to 1 Gigahertz (GHz); and at least oneaerial vehicle that includes a Global Positioning System (GPS) receiverand a transponder, wherein the at least one aerial vehicle is configuredto operate proximate to first and second known coordinates of a propertyline, to determine its coordinates utilizing the GPS receiver, and toprovide its coordinates to the range detector, wherein the rangedetector is further configured to receive first coordinates of the atleast one aerial vehicle in response to the at least one aerial vehicleoperating proximate to the first known coordinates of the property line,to broadcast a first RF signal at f1, to receive a first RF rebroadcastat f2 of the first RF signal from the transponder, and to calculate afirst distance from the range detector to the at least one aerialvehicle based on the first RF signal and the first RF rebroadcast,wherein the range detector is further configured to receive secondcoordinates of the at least one aerial vehicle in response to the atleast one aerial vehicle operating proximate to the second knowncoordinates of the property line, to broadcast a second RF signal at f1,to receive a second RF rebroadcast at f2 of the second RF signal fromthe transponder, and to calculate a second distance from the rangedetector to the at least one aerial vehicle based on the second RFsignal and the second RF rebroadcast, wherein the range detector isfurther configured to calculate a perpendicular distance from the rangedetector to the property line based on the first and second distance,the first and second coordinates of the at least one aerial vehicle, andthe first and second known coordinates of the property line.
 2. Theapparatus of claim 1, wherein: f1 and f2 each have frequency that isselected from a 23 cm band or a 70 cm band.
 3. The apparatus of claim 1,wherein: the range detector is further configured to perform a firstcorrelation between the first RF signal and the first RF rebroadcast, tocalculate a first integer wavelength delay based on the firstcorrelation, and to calculate the first distance based on the firstinteger wavelength delay; and the range detector is further configuredto perform a second correlation between the second RF signal and thesecond RF rebroadcast, to calculate a second integer wavelength delaybased on the second correlation, and to calculate the second distancebased on the second integer wavelength delay.
 4. The apparatus of claim3, wherein: the range detector is further configured to determine afirst phase difference between the first RF signal and the first RFrebroadcast, to calculate a first fractional wavelength delay based onthe first phase difference, and to calculate the first distance based onthe first integer wavelength delay and the first fractional wavelengthdelay; and the range detector is further configured to determine asecond phase difference between the second RF signal and the second RFrebroadcast, to calculate a second fractional wavelength delay based onthe second phase difference, and to calculate the second distance basedon the second integer wavelength delay and the second fractionalwavelength delay.
 5. The apparatus of claim 1, wherein: the first RFsignal and the second RF signal comprise a pulse sequence that ismodulated by a Barker code; the first RF rebroadcast comprises a pulsesequence that is modulated based on the Barker code of the first RFsignal; and the second RF rebroadcast comprises a pulse sequence that ismodulated based on the Barker code of the second RF signal.
 6. Theapparatus of claim 1, wherein: the first and second known coordinates ofthe property line comprise corners of the property line.
 7. Theapparatus of claim 1, wherein: the at least one aerial vehicle comprisesat least one Unmanned Aerial Vehicle (UAV).
 8. A method comprising:operating at least one aerial vehicle proximate to first knowncoordinates of a property line, wherein the at least one aerial vehicleincludes a transponder and a Global Positioning System (GPS) receiver;determining first coordinates of the at least one aerial vehicleutilizing the GPS receiver; broadcasting a first Radio Frequency (RF)signal at a first carrier frequency (f1), wherein f1 that is selectedfrom frequencies of 30 Megahertz (MHz) to 1 Gigahertz (GHz); receiving afirst RF rebroadcast of the first RF signal from the transponder,wherein the first RF rebroadcast is at a second carrier frequency (f2)that is different from f1, wherein f2 is selected from frequencies of 30MHz to 1 GHz; calculating a first distance to the at least one aerialvehicle based on the first RF signal and the first RF rebroadcast;operating the at least one aerial vehicle proximate to second knowncoordinates of the property line; determining second coordinates of theat least one aerial vehicle utilizing the GPS receiver; broadcasting asecond RF signal at f1; receiving a second RF rebroadcast at f2 of thesecond RF signal from the transponder; calculating a second distance tothe at least one aerial vehicle based on the second RF signal and thesecond RF rebroadcast; and calculating a perpendicular distance to theproperty line based on the first and second distance, the first andsecond coordinates of the at least one aerial vehicle, and the first andsecond known coordinates of the property line.
 9. The method of claim 8,wherein: broadcasting the first RF signal and the second RF signal at f1further comprises: broadcasting at a frequency of f1 that is selectedfrom a 23 cm band or a 70 cm band; and receiving the first RFrebroadcast and the second RF rebroadcast at f2 further comprises:receiving at a frequency of f2 that is selected from frequencies fromthe 23 cm band or the 70 cm band.
 10. The method of claim 8, wherein:calculating the first distance further comprises: performing a firstcorrelation between the first RF signal and the first RF rebroadcast;calculating a first integer wavelength delay based on the firstcorrelation; and calculating the first distance based on the firstinteger wavelength delay; and calculating the second distance furthercomprises: performing a second correlation between the second RF signaland the second RF rebroadcast; calculating a second integer wavelengthdelay based on the second correlation; and calculating the seconddistance based on the second integer wavelength delay.
 11. The method ofclaim 10, wherein: calculating the first distance further comprises:determining a first phase difference between the first RF signal and thefirst RF rebroadcast; calculating a first fractional wavelength delaybased on the first phase difference; and calculating the first distancebased on the first integer wavelength delay and the first fractionalwavelength delay; and calculating the second distance further comprises:determining a second phase difference between the second RF signal andthe second RF rebroadcast; calculating a second fractional wavelengthdelay based on the second phase difference; and calculating the seconddistance based on the second integer wavelength delay and the secondfractional wavelength delay.
 12. The method of claim 8, wherein:broadcasting the first RF signal and the second RF signal comprises:modulating a carrier with a Barker code to broadcast a pulse sequence;receiving the first RF rebroadcast comprises: receiving a carrier thatis modulated based on the Barker code of the first RF signal; andreceiving the second RF rebroadcast comprises: receiving a carrier thatis modulated based on the Barker code of the second RF signal.
 13. Themethod of claim 8, wherein: the first and second known coordinates ofthe property line comprise corners of the property line.
 14. The methodof claim 8, wherein: the at least one aerial vehicle comprises at leastone Unmanned Aerial Vehicle (UAV).
 15. An apparatus comprising: a rangedetector configured to transmit RF signals at a first carrier frequency(f1), and to receive RF signals at a second carrier frequency (f2) thatis different than f1, wherein f1 and f2 are selected from frequencies of30 Megahertz (MHz) to 1 Gigahertz (GHz); and at least one aerial vehiclethat includes a Global Positioning System (GPS) receiver and atransponder, wherein the at least one aerial vehicle is configured todetermine its coordinates utilizing the GPS receiver, and to provide itscoordinates to the range detector, wherein the range detector is furtherconfigured, in response to the at least one aerial vehicle operating ateach of a plurality of different locations, to receive coordinates ofthe at least one aerial vehicle, to broadcast an RF signal at f1, toreceive an RF rebroadcast at f2 of the RF signal from the transponder,and to calculate a distance from the range detector to the at least oneaerial vehicle based on the RF signal and the RF rebroadcast, whereinthe range detector is further configured to determine coordinates of therange detector based the distance calculated at each of the plurality ofdifferent locations.
 16. The apparatus of claim 15, wherein: f1 and f2each have frequency that is selected from a 23 cm band or a 70 cm band.17. The apparatus of claim 15, wherein: the range detector is furtherconfigured to perform a correlation between the RF signal and the RFrebroadcast, to calculate an integer wavelength delay based on thecorrelation, and to calculate the distance based on the integerwavelength delay.
 18. The apparatus of claim 17, wherein: the rangedetector is further configured to determine a phase difference betweenthe RF signal and the RF rebroadcast, to calculate a fractionalwavelength delay based on the phase difference, and to calculate thedistance based on the integer wavelength delay and the fractionalwavelength delay.
 19. The apparatus of claim 15, wherein: the RF signalcomprise a pulse sequence that is modulated by a Barker code, and the RFrebroadcast comprises a pulse sequence that is modulated based on theBarker code of the RF signal.
 20. The apparatus of claim 15, wherein:the at least one aerial vehicle comprises at least one Unmanned AerialVehicle (UAV).
 21. A method comprising: operating at least one aerialvehicle at each of a plurality of different locations, wherein the atleast one aerial vehicle includes a transponder and a Global PositioningSystem (GPS) receiver; in response to the at least one aerial vehicleoperating at each of the plurality of different locations, performingthe steps of: receiving, by a range detector, coordinates of the atleast one aerial vehicle utilizing the GPS receiver; broadcasting, bythe range detector, a Radio Frequency (RF) signal at a first carrierfrequency (f1), wherein f1 that is selected from frequencies of 30Megahertz (MHz) to 1 Gigahertz (GHz); receiving, by the range detector,a RF rebroadcast of the RF signal from the transponder, wherein the RFrebroadcast is at a second carrier frequency (f2) that is different fromf1, wherein f2 is selected from frequencies of 30 MHz to 1 GHz; andcalculating, by the range detector, a distance from the range detectorto the at least one aerial vehicle based on the RF signal and the RFrebroadcast; and determining, by the range detector, its coordinatesbased the distance calculated at each of the plurality of differentlocations.
 22. The method of claim 21, wherein: broadcasting the RFsignal further comprises: broadcasting at a frequency of f1 that isselected from a 23 cm band or a 70 cm band; and receiving the RFrebroadcast at f2 further comprises: receiving at a frequency of f2 thatis selected from frequencies from the 23 cm band or the 70 cm band. 23.The method of claim 21, wherein calculating the distance furthercomprises: performing a first correlation between the RF signal and theRF rebroadcast; calculating a first integer wavelength delay based onthe first correlation; and calculating the distance based on the firstinteger wavelength delay.
 24. The method of claim 23, whereincalculating the distance further comprises: determining a phasedifference between the RF signal and the RF rebroadcast; calculating afractional wavelength delay based on the phase difference; andcalculating the distance based on the integer wavelength delay and thefractional wavelength delay.
 25. The method of claim 21, wherein:broadcasting the RF signal comprises: modulating a carrier with a Barkercode to broadcast a pulse sequence; and receiving the RF rebroadcastcomprises: receiving a carrier that is modulated based on the Barkercode of the RF signal.
 26. The method of claim 21, wherein: the at leastone aerial vehicle comprises at least one Unmanned Aerial Vehicle.